1. Field of the Invention
The present invention relates to a spark plug, as well as to a method for manufacturing the same.
2. Description of the Related Art
In recent years, in response to increasing demand for high performance of an internal combustion engine such as an automobile gasoline engine, a spark plug used for providing ignition has been required to enhance ignition performance and to reduce discharge voltage. A reduction in the diameter of a spark portion of a center electrode is effective for enhancing ignition performance and reducing discharge voltage. Thus, many spark plugs have employed a structure in which a noble metal chip is joined to a diameter-reduced distal end of a center electrode so as to form a spark portion. However, recently, in order to enhance fuel economy and to cope with stricter exhaust gas regulations, the trend is toward lean air-fuel mixture (lean burn), and thus ignition conditions are growing increasingly severe. Under these circumstances, even a ground electrode, which is located deeper in a combustion chamber, is subjected to such a trial that a noble metal chip is joined to the ground electrode so as to form a spark portion which protrudes toward the distal end surface of a center electrode from the side surface of the ground electrode, and also the diameter of a distal end portion of the noble metal chip is reduced.
A prior application (Japanese Patent Application Laid-Open (kokai) No. H03-176979) filed by the present inventors discloses a specific structure for reducing the diameter of a noble metal spark portion of a ground electrode. In the spark plug shown in FIG. 2 of the publication, a cylindrical chip of Ir or an Ir alloy having a small diameter is electrically welded (resistance-welded) to an Ni-based electrode base metal directly or via an intermediate layer of Pt-based metal. The chip is subjected to electric welding whereby the chip enters a state in which the chip can be deformed through machining. In this state, through application of pressure, a proximal portion joint-side portion) of the chip is deformed, whereby a flange portion is formed. Formation of the flange portion increases a joining area, whereby the small-diameter chip can be joined with sufficient strength. This is the gist of the prior invention.
However, the subsequent studies have revealed that, in order to join a noble metal to an electrode with sufficient strength, an alloy layer having a certain thickness or greater must be formed between the noble metal chip and a main metal portion of the ground electrode to which the chip is to be joined. Ir, which is used as material for the noble metal chip in the prior invention, has a high melting point equal to or higher than 2,400° C. Thus, in order to form the alloy layer, resistance heating to a considerably high temperature is required. However, an Ni-based electrode base metal and an intermediate layer of Pt-based metal, which constitute the main metal portion of the ground electrode, have melting points far lower than that of Ir (melting point of Ni: 1,453° C.; and melting point of Pt: 1,769° C.). Therefore, when resistance heating to a temperature required for alloying with Ir is performed, as shown in FIG. 13 of the accompanying drawings, the main metal portion of a ground electrode 4 is excessively softened and significantly deformed as compared with a noble metal chip 32′; hence, formation of a normal spark portion becomes very difficult. Also, since the ground electrode 4 is significantly softened, the ground electrode 4 fails to sufficiently receive a compressive deformation force exerted on a proximal portion joint-side portion) of the noble metal chip 32′. As a result, a flange portion 32t is not spread to an expected degree, and most of the flange portion is highly likely to be buried in the ground electrode 4. The thus-obtained spark portion 32 is formed such that, since the flange portion 32t fails to have a sufficient width or is buried in the electrode, its proximal end part protruding from the ground electrode 4 (a protruding proximal end part) is unavoidably surrounded by an exposed surface of an electrode base metal 4, whose melting point is low.
As shown in FIG. 14 of the accompanying drawings, conceivably, as in the case of a center electrode, the Ir-based noble metal chip 32′ may be joined to the ground electrode 4 through laser welding. However, as shown in FIG. 14, when laser welding is used, a weld bead WB, which serves as a joint portion, is formed around a protruding proximal part of the obtained spark portion 32 while having a considerable width (e.g., 0.2 mm or greater). Since a laser beam LB causes heat concentration, the weld bead WB is formed in the following manner: the electrode base metal and the noble metal chip 32′ are fused together, followed by solidification. Thus, the weld bead WB is formed while eroding a considerable portion of the noble metal chip 32′. Since the weld bead WB contains a large amount of electrode base metal, which is, for example, an Ni-based metal, the weld bead WB is significantly lower in melting point than the spark portion 32 formed from an Ir-based metal. That is, a protruding proximal end part of the obtained spark portion 32 is surrounded by the weld bead WB of low melting point.
It must be noted that, when the diameter of the spark portion 32 of the ground electrode 4 is reduced, the following phenomenon is apt to arise. In recent years, in order for an internal combustion engine to enhance fuel economy and to practice lean burn, fuel injection pressure is increased, and employment of a direct-injection-type engine, in which fuel is injected directly into a combustion chamber, is increasingly common. Hence, a gas flow in a combustion chamber is considerably turbulent. When the diameter of the spark portion 32 is reduced in order to enhance ignition performance or other purposes, the area of a distal end surface of the spark portion 32, on which surface sparks land, decreases. As shown in FIG. 15 of the accompanying drawings, when sparking is subjected to a strong, lateral gas flow, the spark SP drifts and runs off the distal end surface of the spark portion 32; as a result, the spark is apt to land on a peripheral electrode surface which surrounds the protruding proximal end part. At this time, if, as shown in FIG. 13 or 14, the peripheral electrode surface is of the electrode base metal or weld bead WB, which are lower in melting point than the spark portion 32, the landing portion is eroded through spark ablation as shown in FIG. 15, thereby causing uneven ablation and thus raising a problem that the life of the ground electrode 4 is terminated at an early stage.